Running diverging closure coerced to fn ptr fails: "invalid use of NULL pointer"

[RalfJung]

The following code fails to run in Miri:

fn main() {
    let f: fn() -> ! = || std::process::exit(0);  
    f();
}

The error is

error: Miri evaluation error: invalid use of NULL pointer
   --> /root/.rustup/toolchains/nightly-x86_64-unknown-linux-gnu/lib/rustlib/src/rust/src/libcore/ops/function.rs:227:5
    |
227 |     extern "rust-call" fn call_once(self, args: Args) -> Self::Output;
    |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ Miri evaluation error: invalid use of NULL pointer
    |
note: inside call to `<[closure@src/main.rs:2:24: 2:48] as std::ops::FnOnce<()>>::call_once - shim` at src/main.rs:3:5
   --> src/main.rs:3:5
    |
3   |     f();
    |     ^^^

I am not entirely sure what is happening; it looks like this has something to do with the shim that is used to coerce non-capturing closures to function pointers.

Cc @eddyb @oli-obk

[eddyb]

Does it have to be -> !? I can't think why that could affect the outcome.

[RalfJung]

I don't understand it either, but replacing ! by () in the example fixes it.

[RalfJung]

There is something weird about the coercion of diverging closures anyway, see rust-lang/rust#66738.

[RalfJung]

Looks like the exception comes from here

https://github.com/rust-lang/rust/blob/04e69e4f4234beb4f12cc76dcc53e2cc4247a9be/src/librustc_mir/interpret/terminator.rs#L80

and then probably

https://github.com/rust-lang/rust/blob/04e69e4f4234beb4f12cc76dcc53e2cc4247a9be/src/librustc_mir/interpret/place.rs#L667

So, we have a function call with a return place, but that return place is actually the callers return place, and the caller diverges. This is okay because the callee also diverges, but we error out when just creating that return place.